Many non-destructive testers for elongated objects such as wire ropes are presently available. Most of these testers comprise electromagnets or permanent magnets which induce a magnetic flux in the objects, and include means for detecting leakage of the flux from these objects. Typically, flux leakages are caused by two different phenomena, namely (a) internal or external defects, also called "local faults", and (b) loss of metallic area due to wear, rust and the like. The former cause of flux leakage is generally localized, while the latter is not.
Current testers operate in the following manner: the elongated object is passed through the tester which is equipped with sensors located between the poles of the magnet, the sensors being adapted to detect flux leakages caused by internal and/or external defects, and by loss of metallic area. Two signals, namely the local fault signal (LF) and loss of metallic area signal (LMA), are therefore generated. An example of such tester is disclosed and claimed in U.S. Pat. No. 4,096,437, which is hereby incorporated by reference.
Problems encountered with current testers are that each signal must be interpreted according to the physical characteristics of the sensing head. Furthermore, the LF signal only serves qualitative purposes because it varies with the spatial derivative of the leakage flux as measured by the LF sensors.
Those problems are summarized by Kuun et al. in "Condition assessment of Winding Ropes" in the Proceedings of the 2.sup.nd International Mine Hoisting Conference in London, UK, Jun. 28-30, 1993, paper 6.2, particularly in page 6.2.2, 2.sup.nd column, 5.sup.th paragraph wherein it is stated that "Existing EM (electromagnetic) instruments do not provide a true indication of steel area at a given cross-section of the rope. Area output is a mean value over a rope length varying from 50 to 360 mm.( . . . ) Finally, EM output is not an absolute value but merely a variation relative to the area of the rope at the location where the output was last set to zero".
Accordingly, even though valuable information is provided with testers presently in use, there is a great need to improve their efficiency in terms of the indication of steel area at a given cross-section of the rope.